Unfortunately, there is no telescope or of any type of other optical instrument that is completely free of image aberrations. The perfect telescope does not exist. Even the eye has some aberrations. But it is always possible to develop optical systems which correct for particular aberrations. Often, it is also a matter the observer’s attitude: i.e. whether he accepts an optical system with certain aberrations, or whether he demands a high-end instrument which produces a virtually perfect image.
Explanations of the most important aberrations in astronomical telescopes can be found on the following pages.
Spherical aberrationSpherical aberration is an aberration that may occur in both the case of lens and mirrors. Here, light rays nearer the optical axis are refracted, or reflected, differently, from light rays further away from it. This means there are different focal planes for the various rays. In the case of a spherical lens, or a spherical mirror, this spherical aberration occurs because the angle of incidence further away from the optical axis is considerable higher than that close to the optical axis. In telescopes, this aberration is seen as a blurring of the image. The aberration is more serious at shorter focal lengths than at longer focal lengths. This aberration can be reduced by use of an aspherically curved lens or a parabolic mirror This means that the angle of incidence are not as high and therefore the light beams come together in one focal plane.
When the Hubble Telescope was first launched into space, it was found that it suffered from spherical aberration and provided blurry images. A pair of ‘spectacles’ had to be made and fitted to it in space to correct for this error.
Chromatic aberrationChromatic aberration is a problem which lens, or refracting, telescopes suffer from. Light strikes the lens elements and is refracted by them - perhaps you can still remember something about this from physics lessons at school. Refraction is essential for the formation of an image. Blue light is refracted more than, say, red light. This means that the different wavelengths have different focal lengths. The refractive index of blue light is greater than that of red light.
If one imagines the effect of this on the formation of an image of an object, then the blue light will be found at a different location than the red light. This means that the image produced is blurred. But not only that, it also means a difference in the magnification of different colours. In plain language, this means that the different image distances for the respective colours cause different image sizes for them. This means the production of annoying colour fringes in the image.
Chromatic aberration can be quite well corrected by use of an achromatic doublet. Here, a positive biconvex lens is combined with a negative lens located behind it with greater dispersion. Thus partially compensates for the chromatic aberration. But even then there is some residual chromatic aberration. This residue is referred to as ‘secondary spectrum’.
Also this secondary spectrum can be corrected, in which you can still inserts an additional lens (usually again a plus lens). In reflecting telescopes occurs no chromatic aberration.
ComaComa is another image error caused mainly by the incident light beam falling obliquely, away from the optical axis. It is often produced by the combination of spherical aberration and astigmatism. Astigmatism is partly due to asymmetric light rays. In the diagram, the light beams generate asymmetric images. This gives rise to stars at the edge of the field of viewing exhibiting distortions that resemble comet tails. These have a fuzzy appearance and cannot be focused.
Telescopes with large aperture ratios tend to suffer particularly badly from coma. These are telescopes with aperture ratios of 1:4 or 1:5 up to about 1:7. In other words, the aberration appears worse with particularly fast optics. Long focal length telescopes, with their smaller aperture ratios (e.g., 1:10), suffer much less from coma. Also, this error can be minimized if the lens is stopped down. It is always possible to use a coma corrector to achieve sharp images with fast optics however.
Astigmatism can be caused by the incident light beam hitting the telescope obliquely (oblique astigmatism). It can also occur due to distortions of the main mirror. But it is often caused by two different curvatures of mirrors or lenses generating different focal lengths. One bundle of rays would then be perpendicular to the other. Astigmatism can be seen in the Airy disk as an image distortion where it is longer in one axis than that perpendicular to it. The aberration can be minimized by stopping down the telescope.
Field curvatureField curvature is related to oblique astigmatism.
The image is formed on a curved surface, rather than on a flat pane, meaning you can never get the image focused simultaneously at both the centre and the edge. Stopping down the lens can also minimize this aberration.